Final answer:
Cholesterol is the short, rigid molecule produced by animal cells that is involved in membrane fluidity.
Step-by-step explanation:
The short, rigid molecule produced by animal cells that is involved in membrane fluidity is cholesterol (A).
The plasma membrane is primarily composed of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, as well as cholesterol in animal cells. Cholesterol helps regulate the fluidity of the membrane, making it more flexible. Therefore, the correct answer is A) Cholesterol.